Gears have long been used in power transmitting machines and mechanisms to increase or decrease an applied torque or the direction in which a torque is applied. Gears are often formed as wheels, worm wheels or linear racks. Elegant gear manufacturing processes have been developed to form the teeth on the wheel or rack structure.
In the case of gear wheels, the basic gear form with unfinished teeth can be, e.g., cast or forged from a blank of a suitable metal alloy. A hardenable steel, such as AISI 5620, is often a material of choice. Teeth are cut into the circumference of the wheel using a hob or other suitable tool. The surfaces of the hobbed teeth are often then further machine finished or polished so that they are precisely shaped and smooth for good engagement with a counter-gear. Grinding, honing and/or chemical polishing are examples of such gear tooth finishing processes.
In the automotive industry, millions of gears are manufactured each year. In one particularly large manufacturing volume application, e.g., planetary gear sets are commonly used in automatic transaxles. Such planetary gear sets contain at least three main components: a sun gear, a carrier assembly with a plurality of planet pinion gears and an internal gear. The sun gear is located at the center of the planetary gear set and has planet pinion gears revolving around it. These planet pinion gears have gear teeth that are in constant mesh with the sun gear. An internal ring gear encompasses the entire gear set. Torque from the engine (input torque) is transferred to the gear set and forces at least one of these components to rotate. Since all three main components are in constant mesh with each other, the remaining components are often forced to rotate as a reaction to the input torque. After input torque passes through a gear set, it changes to a lower or higher torque value known as output torque. In a front wheel drive automobile transaxle, for example, two such suitably sized gear sets are combined and controlled to provide forward drive ratios and a reverse drive. The output torque from the second gear set then becomes the force that is transmitted to the vehicle's drive axles.
The automobile automatic transaxle is but one example of gear set containing mechanisms that must be carefully designed for minimum cost of manufacture and to sustain high loads over a long product life. The need for continuous improvement in automobile design has required engineers to obtain unreduced or greater output from smaller and lighter robust gear mechanisms.
It is observed that the operating life of a power transmitting mechanism such as an automotive automatic transaxle depends significantly on the fatigue life of the gears. There seem to be two main approaches to increasing the fatigue life of a gear set: improving tooth shape and contact area and increasing the hardness of tooth wear surfaces. The improvement of tooth shape and contact area has been accomplished by expensive machining operations and by unselective natural wear-in or run-in of a newly made and assembled set during the first hours of operation of the mechanism. The increase in the tooth hardness has been accomplished by metallurgical surface hardening, e.g., induction surface hardening of a hardenable steel, or carburization and heat treating of an iron or steel alloy, or by application of a thin coating of hard material such as diamond-like carbon, titanium nitride, boron carbide or the like. While such hardened surfaces increase the fatigue life of a gear set, care must be taken to polish the hard surface or it may cause excessive wear of the mating gear surface by abrasion.
The gear making art requires improvements in the manufacture of suitably shaped gear teeth, and the use of hardened gears, and in the assembly of such gears in a robust power transmission mechanism.